The present invention relates to a multi-stage membrane separation and purification process and an apparatus for separating high purity methane gas, wherein a 4-stage membrane recirculation process and operating conditions for the separation and purification of high purity methane gas from biogas are included.
The biogas generated by anaerobic digestion of food waste, organic waste, and livestock wastewater is mainly composed of methane (about 50˜75 volume %) and carbon dioxide (about 25˜50% volume %) and includes some minor impurities such as air (about 0.1 volume %), hydrogen sulfide (about 7,000˜8,000 ppm), and siloxane (about 40 ppm).
Methane, the main component of biogas, is about 20 times more likely to contribute to than carbon dioxide, and is designated as a contributing about 18 volume %, which is about 49 volume %, Followed by carbon dioxide. The major component of the biogas is methane, which is designated as a greenhouse gas whose contribution to global warming is about 20 times higher than that of carbon dioxide and takes about 18 volume % next to carbon dioxide (about 49 volume %). Methane gas has the energy of 5,000 kcal/m3, which is regarded as a renewable energy source that can be recycled.
The method for collecting the biogas and using the collected biogas as resources includes direct burning, electricity production, supply to city gas, and use as automobile fuel, etc. Various application methods are under development according to the generation background of methane gas and its economics. Among them, the highly economical method with high energy efficiency is the method for preparing high purity methane gas fuel having at least 95% purity usable as city gas or automobile fuel through the purification process increasing energy content, that is the methane concentration, with high purity from biogas. This method is more economical than the method using it for the production of electricity, so that world-wide countries including Sweden and Germany are attempting to use such methane gas as a high purity fuel. The high purity methane gas can be applied to any conventional city gas apparatus and natural gas vehicle without exchanging equipments. Therefore, this gas is expected as a next generation clean bio-energy, and the advanced countries especially advanced with the renewable energy including Sweden and Germany establish national plans for using biogas instead of natural gas.
Various techniques usable for the separation process and plant for obtaining high purity biogas, and the operation conditions thereof have been developed. The method to produce high purity bio-methane is composed of two techniques: pre-treatment technique to eliminate impurities remaining in the biogas such as siloxane, ammonia, hydrogen sulfide, and moisture from the biogas; and de-carbon dioxide separation technique to separate carbon dioxide and methane from each other. The de-carbon dioxide separation technique is exemplified by cryogenics characterized by direct separation at a low temperature, physical or chemical absorption using water or amines, pressure swing adsorption using zeolite or carbon adsorbent, and membrane separation using methane-specific polymer separation membrane.
The development and commercialization of the biogas high purity purification technique has been led by USA and EU countries. The representative companies retaining biogas purification technique are Malmberg, Purac, and Flotech in Sweden and Prometheus Energy in USA which use absorption using water, polyethylene glycol, or amines as an absorption liquid; Evonik in Germany, Air-liquides in France, and Acrion Technology in Austria which use separation membrane method using polyimide membrane or polysulfone membrane; and Schmack and Carbotech in Germany and Xebec in Canada which use adsorption using zeolite or carbon adsorbent. In addition, the hybrid process of membrane separation, cryogenics, and absorption is also researched and developed.
As an example of absorption method, Korean Patent Publication No. 10-2010-0037249 describes the purification system and method of high purity biogas. Precisely, the purification system and method of biogas above comprises the pre-treatment part to eliminate moisture, hydrogen sulfide, and siloxane in order for the biogas generated in the anaerobic digestion part to be used as a gas fuel; and either the gas adsorption part to eliminate carbon dioxide by using a adsorbent or the gas absorption part to absorb and dissolve carbon dioxide by using a absorbent.
Korean Patent Publication No. 10-2012-0083220 describes the methane recovering method and apparatus. Precisely, the method above comprises the steps of eliminating siloxane in the biogas by using a adsorbent; eliminating hydrogen sulfide in the course of reaction removal process wherein hydrogen sulfide is reacted with metal oxide to turn into metal sulfide and removed as the form of metal sulfide; capturing copper oxide in the course of capture process wherein oxygen in the biogas is reacted with copper-zinc oxide to turn into copper oxide and then copper oxide is captured; and concentrating methane in the course of concentration process wherein carbon dioxide in the biogas is separated and then methane is concentrated by pressure swing adsorption method.
However, the methane purification methods described above are based on carbon dioxide absorption process or PSA adsorption process. Therefore, the installation cost of the plant is high and the process operation cost is also high. Besides, the small-scale apparatus configuration is not possible, the purification efficiency is low, and the process is complicated and consumes a lot of energy.
It was believed that the membrane separation was more suitable for the domestic biogas purification facilities and it was easy to maintain and yield high purity methane. Unlike other separation methods, the membrane separation method is operable in dry process, which is advantageous in winter; is pro-environmental because of not using a toxic absorbent; costs less for the plant and operation; and is easy for scale-up and scale-down. So, the membrane separation method is expected to occupy a unique position in future biomethane purification technology.
The two most important targets in the membrane separation process are methane concentration and recovery rate. In the single-stage membrane separation process, the recovery rate is usually 60˜75%. To increase the recovery rate of methane, a two-stage membrane separation process wherein the separation membrane is connected in two stages, the permeate of the first stage separation membrane is incinerated, and the permeate of the second stage separation membrane is recirculated is under development together with a three-stage membrane separation process wherein the permeate of the first stage separation membrane of the two-stage membrane separation process passes through the third stage separation membrane and methane gas that does not pass through the second stage separation membrane is recirculated.
Korean Patent Publication No. 10-2011-0037921 describes a low temperature biogas separation method as an example of the single-stage membrane separation process. In this method, the biogas generated in the anaerobic condition is changed into the compressed biogas of 7 bar through desulfurization process, siloxane removal process, compression process, and moisture removal process. Then, methane is purified from the compressed biogas through the single-stage membrane separation process using a polystyrene hollow fiber membrane module.
As a method to separate and collect methane and carbon dioxide from biogas through the membrane separation like the above, the single-stage membrane separation process has been used. However, the recovery rate of methane contained in the biogas is only about 70% or less with this method, indicating the methane collection is inefficient and the additional methane recovery process is required. And the energy consumed in the system is excessive, resulting in a low energy efficiency of the system.
To solve the problems above, the multi-stage membrane separation process has been developed to purify methane from biogas.
As an example of the multi-stage membrane separation process, Japanese Patent Publication No. 2007-254572 describes a methane concentration two-stage system and an operation method of the same. In this method, a mixed gas is provided to the first separation membrane and the non-permeated gas is provided to the other separation membrane in the back with increasing pressure. Carbon dioxide is passed through the second separation membrane, then. Accordingly, high concentration methane gas is collected. At this time, the carbon dioxide permeable membrane is preferably the DDR type zeolite membrane, according to the patent document.
Japanese Patent Publication No. 2008-260739 describes a two-stage methane concentration device and a methane concentration method. Precisely, this method comprises the following steps; a mixed gas is passed through the first separation membrane made of inorganic porous materials; and the non-permeated gas is passed through the second separation membrane made of inorganic porous materials. At this time, the separation membrane is made of inorganic porous materials.
US Patent Application Publication No. US 2004/0099138 describes a membrane separation process. According to this method, at least 98% methane was recovered from landfill gas by using a carbon dioxide absorption tower and a two-stage membrane separation process. The landfill gas was supplied to the two-stage membrane separation process through the first compression process, the dehumidification process, the second compression process, the heat exchange process and the carbon dioxide absorption process. The provided gas was compressed to 21 bar in the first compressor and compressed to 60 bar through the second compressor and heat exchanger to facilitate the operation of the carbon dioxide adsorption tower and heated to 30° C.
The gas compressed through the permeation part of the first separation membrane comprising 90% carbon dioxide and 10% methane and impurities was recirculated to the upper part of the carbon dioxide absorption tower. The gas that had passed through the permeation part of the second separation membrane was provided to the second compressor to increase the recovery rate of methane. In addition, Ecrion technology using the polyamide-imide membrane developed by Air Liquide, France, is an example of the two-stage recirculation membrane separation process. The two-stage membrane separation processes informed from the prior arts use various separation membranes. The disadvantage of the processes above is that the recovery rate of the purified methane having at least 95% purity is less than 90%.
Japanese Patent No. 2009-242773 describes a three-stage membrane separation process. Precisely, this method includes a methane concentration device wherein carbon dioxide is separated from the mixed gas composed of methane gas and carbon dioxide and methane gas is concentrated. This device includes the first compressor compressing methane gas by the separation membrane letting carbon dioxide pass through first from the mixed gas; the second compressor to compress methane gas from the non-permeated gas remaining in the first compressor by using the separation membrane letting carbon dioxide pass through first; and the recovery device to collect methane gas from the gas that passed through the first compressor by using the separation membrane above. The separation membrane herein is preferably a polyimide membrane according to the patent document. However, the area ratio is similar between the first and the second stage separation membrane and the area of the third stage separation membrane is simply smaller than the first stage separation membrane, suggesting that the process condition including temperature or membrane area is not concrete. Therefore, this method is considered not so promising for the commercialization requiring high methane purity and recovery rate.
A three-stage membrane separation process was first developed in 2010 and commercialized by Evonik Co., Germany. This company has been actively studying on the membrane separation process with the polyimide (P84) hollow fiber membrane developed by themselves since 2008. As a result, a three-stage membrane separation recirculation process is patented and commercialized. Particularly, the said company has a patent of a three-stage process wherein the permeate is recirculated from the second stage in the first-stage and second-stage in-stream permeate to the stepwise arrangement and recompression by the retentate (PCT/EP2011/058636). The separation membrane used in this method is made of a material comprising methane/carbon dioxide ratio of at least 35. According to the reports about the three-stage membrane separation process made by Evonik Co., the polyimide membrane displays 50% higher selectivity than the polysulfone membrane.
Thus, according to the reports methane yield from the gas comprising methane concentration of about 98% reaches 99% through the three-stage process at a high pressure of 16˜20 bar. The polysulfone membrane selected in the example of the present invention displayed at least 300% of recycling rate but the polyimide membrane showed up to 50% of recycling rate.
As shown in Table 1 below, the conventional polyimide material is expensive, so that the membrane production with this material costs high. The selectivity of carbon dioxide/methane of this material is relatively high (about 50) and the carbon dioxide permeability is low (less than several barrels). So, in order to save the separation membrane, a high pressure operation condition is required. However, for such a high pressure operation, costs for the plant equipment including compressor, piping, measurement equipment, and membrane as well are high and also energy consumption for the high pressure operation is also increased. Besides, the location for the plant is limited with considering the risks of plant mal-functioning or methane gas explosion. There is another disadvantage that the cost of replacing the membrane due to contamination of the membrane during the operation is high, which makes it difficult to develop the market.
As shown in Table 1, such membranes as polysulfone membrane, cellulose membrane, acetate membrane, and polycarbonate membrane are comparatively less expensive than polyimide membrane. Even though they have low carbon dioxide/methane selectivity, they show high carbon dioxide permeability. So, the membrane module thereof is not expensive and the permeability is high, suggesting that the required number of membranes for the device is small, indicating the plant construction costs less and the membrane replacement for exchanging the contaminated membrane costs less, too. If a separation membrane made of a polymer separation membrane with as low selectivity as 20 or under is used for the process, the volume of recycled gas to yield high purity methane would be too big and accordingly energy consumption will be huge. On the other hand, if polyimide having as high selectivity as at least 50 is used as a membrane material, which usually displays very low permeability, the yield of high purity methane would be small but the volume of gas for recycling would be bigger, suggesting that many separation membranes and high pressure operation condition are required, and as a result the scale of the device required in the process would be big. If only the optimum operation condition that can recover high purity methane with high yield can be secured by using a high permeability material, polysulfone, cellulose, acetate, and polycarbonate having a medium carbon dioxide/methane selectivity of 20˜34 can be preferred as a separation membrane material. In addition, the separation membrane having high carbon dioxide permeability of 100 GPU˜1,000 GPU developed as an asymmetric hollow fiber membrane or composite flat membrane would be also preferred as a separation membrane. Polysulfone (PS) having slightly lower selectivity but higher carbon dioxide permeability than polyimide and having high resistance against plasticization of carbon dioxide by high supply side pressure would be preferred, too. Particularly, the price of polysulfone as a separation membrane is only 1/20 of the price of polyimide, so that it is advantageous for the membrane replacement required when the separation membrane is damaged by hydrogen sulfide, compaction, and contamination, etc. Unlike the high pressure operation process of Evonik Co., polysulfone having high permeability is usable under the low pressure operation condition, suggesting that the cost of separation membrane and piping can be saved and the operation is safe, and the cost for compressor and other related equipments can also be saved.
TABLE 1PCO2PCO2/Polymer Type(Barrer)PCH4Polytrimethylsilylpropyne331002.0low selectivitySilicone rubber32003.4highNatural rubber1304.6permeabilityPolystyrene118.5Polyamide(Nylon 6)0.1611.2Poly(vinyl chloride)0.1615.1Polycarbonate(Lexan ™)10.026.7mediumPolysulfone4.430.0selectivityPolyethyleneterephthalate(Mylar ™)0.1431.6mediumCellulose acetate6.031.0permeabilityPoly(ether imide)(Ultem ™)1.545.0high selectivityPoly(ether sulfone)(Victrex ™)3.450.0lowPolyimide(Kapton ™)0.264.0permeability1 Barrer = 10−10 cm3 · cm · cm−2 · s−1 · cmHg−1*PCO2 = carbon dioxide permeability*PCO2/PCH4 = carbon dioxide/methane selectivity*Basic principles of Membrane Technology (second edition), Kluwer Academic Publishers, Marcel Mulder.
As an example of the multi-stage membrane separation process patented in Korea, Korean Patent No. 10-1086798 describes a separation method for high purity methane gas from landfill gas and a separation apparatus for methane gas. Precisely, the method comprises the pretreatment step which is similar to the above but can be performed at a low pressure and a low temperature (7˜15 bar, 10˜50° C.); and the combination of two-stage membrane separation process and pressure swing adsorption process for the recovery of high purity methane. However, the use of the method above is limited to landfill gas, which means it targets the gas containing nitrogen and oxygen which are not included in biogas, so that the operation condition is different. In particular, this method includes PSA treatment process targeting the gas remaining not permeated through the separation membrane. So, it is not suitable for the process of biogas that does not contain nitrogen or oxygen but contains a low concentration of hydrogen sulfide and a high concentration of methane.
Korean Patent No. 10-1100321 describes a system for purifying/upgrading and compressing biogas. Precisely, the biogas produced in the anaerobic digestion biogas facility was upgraded by using a siloxane removal device, a desulfurization device, a compression device, a gas heater, and a two-stage membrane separation device, according to the system above. In the system, the provided gas was compressed to 10 bar through the compression device, which was heated to 50° C. by using the gas heater before it would be supplied to the membrane separation device. However, such a high temperature operation condition accelerates plasticization of a polymer membrane, resulting in the low methane/carbon dioxide selectivity. In addition, the ratio of upper part pressure/lower part pressure is low but the supply side temperature is too high. Therefore, it is considered that the feasibility is low.
Further, Korean Patent Publication No. 10-2014-0005846 describes a process for separation of gases. In this method, a gas separation module having the selectivity of at least 35 was used to yield high efficiency even at a high pressure of 9˜75 bar at the supply side and 3˜10 bar at the permeation side. The pressure and selectivity dependent separation results are described in the patent document and the disadvantages of the membrane separation processes having various stage patterns from the single-stage to the three-stage are also described therein. The process above is running at a high pressure so that energy and plant costs are high, which is another disadvantage.
Korean Patent No. 10-1327337 describes a multi-stage membrane separation system and method thereof for production of biomethane and recovery of carbon dioxide. Precisely in this method, the membrane separation structure was formed in multi-stage. Carbon dioxide recovered primarily through the separation membrane was passed through the separation membrane again to give high purity carbon dioxide. In particular, the temperature of the compressed gas was regulated in the range of 20˜30° C. to prevent condensate generation after eliminating moisture. Then, pressure was applied to biogas at 10˜20 bar. However, as shown in FIG. 3, the recirculation process is described at the rear end of the compressor. Therefore, it is predicted that efficient process operation will be technically difficult.
The described methods above for the purification of methane through two-stage or three-stage processes have disadvantages of high operation temperature or pressure, high area ratio, high upper part/lower part pressure ratio, in addition to the high price of a membrane material because the system uses a high price polymer material or an inorganic membrane material having a high selectivity. Only one or two operation conditions among the above-mentioned process conditions were considered and the results of those operations were not precisely described, suggesting that the feasibility of the process seems not easy due to the unsatisfactory yield of the process.
In the case of purifying biogas with variable methane concentration, particularly in the case of purifying biogas having a low concentration of methane, it is more difficult to obtain high purity methane.
In the course of study about the method for separating methane gas by membrane separation, the present inventors developed a method for separating high purity methane gas having at least 95% purity with a high recovery rate of at least 90% by performing a three-stage membrane separation process using a polymer separation membrane prepared with a low price polymer material such as polysulfone characterized by high carbon dioxide permeability and relatively high methane/carbon dioxide selectivity that is lower than those of polyimide but comparatively higher than others, wherein the conditions such as operation temperature, operation pressure, and pressure ratio of upper part/lower part were all optimized to increase the specific selectivity of the polymer separation membrane most and also the total area ratio and stage area ratio of the gas separation membrane were optimized. The present inventors also developed another method for the separation of high purity methane gas having at least 95% purity by using a four-stage membrane separation process using an inexpensive polymer separation membrane whose module per unit area costs is low due to the excellent workability, leading to the completion of the present invention.